It is known to control the air charge provided to a vehicular engine in order to achieve a torque response in line with a torque request provided to the vehicle by, for example, a driver. The fresh air charge provided to an engine is largely a function of the valve lift of the inlet valves and the pressure of the engine intake manifold, together with partial pressure from the exhaust gas residual fraction which can be altered by the both the inlet and exhaust valve timing.
In some engine systems the required air charge is achieved through variation of the pressure in the engine intake manifold whilst the valve lift of the inlet valves is fixed and predetermined. Specifically, the valve lift is dictated solely by the cam profile and thus is not varied cycle-on-cycle. The pressure of the engine intake manifold is varied by controlling the throttle valve, which controls the flow of air into the engine intake manifold.
These engine systems have high pumping losses, particularly at idle or at small throttle valve openings, due to a high pressure differential across the inlet valve which in turn requires the piston to do more work to draw the air from the engine intake manifold into the engine's cylinders.
In other engine systems the throttle valve is controlled to maintain a fixed pressure within the engine intake manifold or else these engine systems do not comprise a throttle valve at all. Variation in the air charge provided to the engine is achieved by controlling the valve lift of the inlet valves. The valve lift of the inlet valves is not fixed. The valve lift is varied to achieve the required engine charge and the control of this variation assumes a fixed engine intake manifold pressure.
In order to achieve the required air charge using these engine systems the inlet valves may remain open for a longer or shorter period of the combustion cycle than is optimal. As a result, the air charge motion within the engine's cylinders is sub-optimal and this in turn decreases the combustion efficiency, resulting in a decrease in the fuel economy.
It is an aim of the present invention to address disadvantages of the known engine systems and provide a more efficient engine without compromising the responsivity to a torque request.